the present invention relates to the destruction of polychlorinated biphenyls (PCBs). Such biphenyls have been used in the past on a large scale in electrical equipment as fire resistant, heat stable dielectric material and as a coolant. However, these compounds have been identified as being environmentally hazardous, and manufacture and sale of them is now prohibited in most countries. Fluids contaminated with more than 2 ppm PCBs may in some jurisdictions be considered to require special handling and may not be disposed of in ordinary disposal sites.
A number of methods are known for removal or destruction of PCBs and like polyhalogenated biphenyls. For example, known methods include absorption in swellable solid polymers, destruction of PBBs (polybrominated biphenyls) in a thermite reaction, reaction with a mixture of polyalkylene glycol alkyl ether and alkali metal hydroxide, extraction into methanol, combustion in a diesel engine or similar combustor, decomposition in molten salts, and dehalogenation employing finely divided molten sodium dispersion.
The latter method, namely dehalogenation with molten sodium dispersion, offers the advantages that it is relatively easily controllable and under carefully maintained conditions can be operated without substantial risk of escape of PCBs or similar materials from the system, has relatively low operating and energy consumption costs, and is capable of substantially complete destruction of the PCBs or similar materials.
The present applicants have, however, determined that known processes of dehalogenation using sodium dispersion consume quantities of sodium that are greater than are desirable, are less energy efficient than are desirable and produce undesirable byproducts which may tend to be emitted from the system unless special precautions are taken. These problems arise because typically insulating fluids based on PCB and similar materials-contain substantial quantities of chlorobenzenes, which are normally added to reduce the viscosity of the mixture. For example askarel, usually a PCB based electrical insulating fluid, usually contains about 30 to about 70% by weight of mixture of different PCBs and the balance a mixture of different di-, tri- and tetrachlorinated benzenes. On reaction with the sodium dispersion, the halogenated benzene species are reduced to benzene, with concomitant oxidation of the sodium to sodium halide and hence their presence adds to the consumption of sodium in the system. In addition, the presence of halogenated benzene in the reaction mixture increases the total volume of material to be reacted and hence increases the energy consumption and operating costs of the dehalogenation process. Moreover, the dehalogenated product, namely benzene, is toxic and is a volatile liquid which tends to be evolved in vapour form from the reaction mixture under the elevated temperatures normally reached in the exothermic dehalogenation process. Accordingly, with known processes, special absorptive filters or other arrangements should be employed to avoid releases of toxic benzene vapour.
The present invention in one aspect relates to a procedure for removing chlorobenzenes from askarels and like mixtures of PCBs and chlorobenzene in order to obtain a mixture with reduced chlorobenzene content which can be more efficiently subjected to PCB destruction employing reaction with sodium dispersion. Further, the invention relates to improvements in the PCB destruction processes whereby the mixture impoverished in chlorobenzenes can be rapidly and efficiently reacted with the sodium dispersion, without the need to add reaction catalysts, to achieve a product substantially free from PCB. Further, the invention relates to improvements in the techniques employed for dispensing a measured quantity of sodium dispersion.
In one aspect, the present invention is based on the finding by the inventors that by conducting fractional distillation of askarels and like mixtures of PCBs and chlorobenzenes, the quantities of chlorobenzenes present in such mixtures can be significantly reduced, in that substantial quantities of the more volatile chlorobenzenes can be efficiently distilled out, and that it is readily possible to separate off, in substantial quantities, a distillate which contains less than 2 ppm PCBs and is thus safely eligible for disposal without needing to take the stringent precautions necessary for disposal of PCBS. Such separation can be effected at relatively high rates of throughput without requiring the use of large or expensive distillation apparatus.
Accordingly, the present invention provides a method for the destruction of polychlorinated biphenyls (PCBs) wherein said PCBs are present in the form of a mixture with chlorinated benzenes, comprising subjecting said mixture to fractional distillation, separating a distillate rich in chlorinated benzenes and containing less than 2 ppm PCBs, collecting a bottoms poor in chlorinated benzenes and rich in PCBs, and reacting said bottoms with a dispersion of sodium particles to reduce substantially all said PCBs to biphenyl.
As is, of course, well understood by those skilled in the art, in the course of fractional distillation, separation of more volatile from less volatile components takes place within a fractionating column through which vapours rise, and a certain amount of liquid, termed reflux, descends. The vapours usually originate from a heated reboiler at the bottom of the column and the reflux liquid usually originates from condensation of vapours at a condenser at the upper end of the column. As the hot vapours from the reboiler come into contact with cooler descending reflux liquid, there is a progressive enrichment of the more volatile constituents upwardly through the column and progressive enrichment of the less volatile constituents downwardly through the column. The column may be, for example, a packed or differential stage contactor column or may be a plate, tray or finite stage contactor column, and the distillation may be conducted continuously or in batch mode. Preferably, the distillation is conducted continuously by reason of greater efficiency of operation.
Askarel and like mixtures currently stored and requiring disposal and destruction contain a wide variety of components. Some of these mixtures contain substantial quantities of mono-, di- and trichlorinated biphenyls. Such mixtures normally exist as neat PCBs, since the PCBs having lower degrees of chlorination, namely having up to about three chlorines atoms per molecule, tend to have adequate low temperature flow characteristics or viscosities without requiring addition of chlorobenzenes or like diluents in order to thin the mixture. Other askarels, however, contain substantial quantities of tetra- to nonachlorinated or more highly chlorinated biphenyl species and these normally exist in the form of a mixture with chlorobenzenes, the total concentration of the chlorobenzenes varying somewhat depending on the nature of the PCBs and on the application for which the askarel was intended. Further variability in the composition of the askarels is added by the fact that the chlorobenzenes employed as viscosity-reducing diluents range from monochlorobenzene to hexachlorobenzene making a total of twelve congeners including the various isomers of di-, tri- and tetrachlorobenzene.
Advantageously, in the present invention the fractional distillation is applied selectively to mixtures having relatively low contents of mono, di- and trichlorobiphenyls, relatively high contents of chlorinated benzenes and relatively low contents of tetra- or more highly chlorinated benzenes. If the content of lower chlorinated biphenyls is too high, there tends to be greater difficulty in significant reduction of the content of chlorobenzenes without carry over of any substantial quantity of PCBs in the distillate. If the total content of chlorinated benzenes is excessively low, significant reduction in the quantity of PCB mixture cannot be achieved, and if the content of tetra- or more highly chlorinated benzenes is excessively high, there again tends to be difficulty in distillation off of a significant proportion of the chlorobenzenes without carry over of any substantial quantity of PCBs into the distillate. Desirably, in the most preferred forms of the present invention about 75% to about 95% by weight of the chlorobenzenes are removed from the askarel starting material, based on the total weight of chlorobenzenes present in the mixture.
Preferred ranges of compositions to which the distillation procedure according to the present invention is applied are shown in Table 1.
In order to achieve a desired degree of separation in the fractionation column a certain range of the number of theoretical stages together with a certain range of reflux ratios are preferred. As is well understood by those skilled in the art, a theoretical stage refers to a contacting stage at which equilibrium is attained between the liquid and vapour. The number of theoretical stages in column is, as is well understood by those skilled in the art, dependent on the dimensions and geometry of the column and on form of construction of the trays or plates or on the nature of the packing material in the case of a packed column. The reflux ratio is the ratio of the volume of distillate returned from the condenser to the column to the volume of distillate withdrawn from the condenser.
As the reflux ratio is increased, the number of theoretical stages required for a given separation decreases. Generally, however increase in the reflux ratio beyond a certain point may tend to reduce the rate of throughput of distillate undesirably, as well as increasing the operating costs and energy costs resulting from increased demand for heating at the reboiler and for coolant at the condenser, while increase in the number of theoretical stages beyond a certain point may tend to increase the dimensions of the column undesirably and thus tend to increase the costs also. Preferably, in the present process a column having about 10 to about 40, more preferably about 20 to about 30, theoretical stages is employed, and a reflux ratio about 1 to about 5, more preferably about 2 is employed.
Preferably, the fractional distillation is conducted under subatmospheric pressure. This has the advantage that the lower the pressure, the lower temperature of operation of the reboiler and of the condenser, thus tending to save energy costs and increasing the intrinsic safety of operation of the column. In addition, it has been found that with the above described askarel composition, operation at reduced pressure is thermodynamically favourable, apparently because of a non-linear relationship between temperature and the vapour pressures of the components of the compositions. For example it may be desired to separate 94% of the chlorobenzenes from a mixture having relatively high contents of tetrachlorobenzene and mono-, di- and trichlorobiphenyls by weight (30% trichlorobenzene by weight, 10% tetrachlorobenzene, 0.6% monochlorobiphenyl, 9.6% dichlorobiphenyl, 29.4% trichlorobiphenyl, and 20.4% tetrachlorobiphenyl), with less than 2 ppm carry over of PCBs. At an operating pressure of 100 mm Hg a column with 30 theoretical stages is necessary to achieve this separation, with operating temperatures in the column ranging from 150 to 250xc2x0 C. Under equivalent conditions, operating at 10 mm Hg pressure, the same separation is achieved using 25 theoretical stages and operating temperatures of 100 to 200xc2x0 C. However, excessively low pressures tend to create mechanical problems since it is difficult or expensive to construct the apparatus to be capable of withstanding large pressure differences. Preferably, the operating pressure is in the range about 5 mm Hg to about 40 mm Hg, more preferably about 5 to about 20 mm Hg.
As indicated above, preferably the fractionation is carried out continuously. In such case it is highly preferred that the feed of the mixture to be distilled be supplied to the column at an intermediate point adjacent the lower end thereof. For example, in the case of a column having 25 theoretical stages, the feed is supplied at a point corresponding to about 5 theoretical stages from the bottom of the column. If the feed is made at or adjacent to the upper portion of the column, there is a tendency for breakthrough of excessive quantities of PCBs to the distillate, especially when feeds containing appreciable quantities of mono-, di- or tri-chlorinated biphenyls. If the feed is made at or below the lower end of the column, heating of the feed to a temperature at or above the temperature of vaporization of the feed is necessary, since it is usual to preheat the feed to the steady state temperature of the column at the point of input in order to avoid disturbance of the steady state temperature profile. However, as will be appreciated there are considerable difficulties and hazards involved in working with vaporized feeds outside the confines of the fractionation column.
In the preferred form, PCBs to be destroyed, such as the bottoms impoverished in chlorobenzenes obtained from the above described distillation are reacted with the sodium dispersion by contacting a measured batch of said bottoms isolated in a reaction vessel, at a temperature of about 120xc2x0 C. to 160xc2x0 C., and having a concentration of PCBs of about 15,000 to about 80,000 ppm, with a measured batch of said sodium dispersion containing at least a weight of sodium stoichiometrically required to react with the chlorine in said PCBs while vigorously agitating the reaction mixture in order to obtain an autocatalytic reaction.
It may be noted that the reaction is preferably conducted as a batch process since control of the quantities of the reactants and of the reaction conditions is greatly facilitated in batch processing, rather than continuous processing, enabling substantially complete destruction of the PCBs. For example with batch processing it is possible to maintain control of the reaction until, as indicated by testing of samples withdrawn from the reactor, no PCB is detectable in the reaction mixture. With continuous processing control of the reaction is not easily maintained and since at steady state a gradient or profile of reactant concentrations is achieved, at least in theory, in order to obtain a zero concentration of PCBS in the output stream an infinitely long reaction vessel may be required.
With the temperatures and PCB concentrations noted above in the preferred form of the reaction, under vigorous agitation, an autocatalytic reaction can be achieved in which there is a rapid and sustained rise in the rate of reaction, which proceeds vigorously exothermically, without the need for addition of any catalyst to sustain the reaction rate. In such case, once the reactants have reached a temperature at which reaction commences, it is normally necessary to cool the reactants, for example by flowing coolant through a cooling coil with which the reaction vessel is equipped in order to avoid excessive temperature rise, leading to such problems as possibly reaching of the flash point of the reactant mixture or polymerization of, for example, mineral oil constituents of the reaction mixture. Desirably the content of PCBs in the reaction mixture is not substantially in excess of about 80,000 ppm, since with concentrations greatly in excess of this value, depending on the degree of chlorination of the PCBs, there tends to be excessive heating of the reaction mixture as a result of excessively vigorous exothermic reaction lending to the problems mentioned above as well as generation of excessive quantities of solid NaCl, which leads to problems in pumping and agitation of the reaction mixture and also producing excessive quantities of biphenyl or polybiphenyl which is a solid at room temperature and can also lead to problems of dealing with the reaction mixture.
In the preferred form the process is operated with only a small excess of sodium over the stoichiometric amount. In view of the efficiency of reaction, it has been found that satisfactory destruction of the PCBs can be achieved in relatively short reaction times with a molar ratio of sodium to chlorine of about 1.2 to about 4, more typically about 1.5.
Once the PCB destruction reaction has commenced to decline, small amounts of water are preferably added to reaction mixture, preferably in an amount of about 2 to about 10% based on the weight of sodium added. The addition of water is believed to generate free radicals, and is found to complete the destruction of PCBs within a shorter time.
A further aspect of the present invention relates to a procedure for obtaining and dispensing a measured batch of a sodium dispersion. Examples of the dispersions to which the invention may be applied include dispersions containing about 20% to about 60% by weight sodium, the balance comprising an inert oil such as mineral oil. In preferred examples, the particle size of the sodium in the dispersion is about 2 to about 10 microns, more preferably about 5 microns. Sodium dispersions of the above type are available from various supplies of laboratory and industrial chemicals. In the past attempts have been made to meter a quantity of sodium dispersion using metering pumps. However, the sodium is highly adherent and tends to adhere to propeller vanes or other moving elements of the pumps leading to disabling of the pump. Cleaning of the pump is a hazardous procedure because of the danger of burns resulting from contact with the sodium or of explosion of the sodium. Further attempts have been made to measure quantities of sodium by using vessels supported on weighing balances, into which vessels the sodium dispersion is supplied. However this is a cumbersome procedure since normally the vessel is equipped with various supply and withdrawal lines and variations in the quantities of reactants remaining in the lines can give rise to uncertainty in the result. Attempts have also been made to measure quantities of sodium dispersion using vessels equipped with level indicating devices such as floats. However, the sodium tends to adhere to the floats rendering them inoperative.
In the present invention there is provided a method for obtaining a measured batch of a sodium dispersion, comprising the steps of supplying said dispersion under pressure into a closed vessel containing an inert gas and equipped with a pressure indicator until the pressure indicator indicates a predetermined pressure, interrupting the supply of said dispersion, and expelling the dispersion from the vessel under pressure exerted by the inert gas compressed within the vessel.
This procedure avoids the need for any form of weighing apparatus and enables measuring of the sodium dispersion without contact with any moving parts such as floats or pump elements.